<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
         .box{
             background-color: #333;
             font-size: 30px;
         }
         .box:hover{
             transition-property: background-color;
             transition-duration :3s;
             transition-timing-function: linear;
             background-color: red;
         }
    </style>
</head>
<body>
    <div class="box">hello</div>
</body>
</html>